There is already a "gun class guild" in the game. It shares the same building with the Marauder guild in the Coral Tower of Upper Limsa Lominsa. Has been there from 1.0 iirc.
Inside you will find a gun toting guild leader and a bunch of npcs practicing with their guns in the main area and the target practice area of the basement.
Also
Merlwyb Bloefhiswyn the leader of the city state wields guns as well.
E3 Interview-
Machinist!
MMORPG: What’s your favorite Final Fantasy class that is not in Final Fantasy XIV.
Yoshi - P: I really like Cecil’s story from Final Fantasy IV, so the Dark Knight. From Final Fantasy Tactics, the Machinist.